1. Field of the invention
The present invention relates to a method of manufacturing a capacitive element in which a first electrode and a second electrode oppose each other through a capacitive dielectric film.
2. Description of the Related Art
FIGS. 1A to 1D show the first related art, exemplifying a method of manufacturing a so-called simply stacked type capacitive element constituting the memory cell of a DRAM (Dynamic Random Access Memory). In this first related art, an SiO.sub.2 film 12 and an Si.sub.3 N.sub.4 film 13, which serve as interlayer dielectric films, are sequentially deposited on an Si substrate 11 having a transistor (not shown) formed thereon, as shown in FIG. 1A.
Subsequently, as shown in FIG. 1B, a contact hole 14 for a lower electrode is formed in the Si.sub.3 N.sub.4 film 13 and the SiO.sub.2 film 12 so as to reach the diffusion layer (not shown) of the transistor. A polycrystalline Si film 15 is deposited, and the polycrystalline Si film 15 is doped with phosphorus 16 or the like by, e.g., ion implantation. Thereafter, as shown in FIG. 1C, the polycrystalline Si film 15 is processed into the shape of a lower electrode.
Next, as shown in FIG. 1D, a capacitive dielectric film 17 is formed on the surface of the polycrystalline Si film 15 and the like. To form this capacitive dielectric film 17, first, the native oxide on the surface of the polycrystalline Si film 15 is removed by hydrofluoric acid or the like. Thereafter, the resultant structure is nitrided by rapid thermal anneal in an NH.sub.3 atmosphere at 850.degree. C. for one minute, thereby forming an Si.sub.3 N.sub.4 film on the surface of the polycrystalline Si film 15.
After an Si.sub.3 N.sub.4 film is deposited, an SiO.sub.2 film is formed on the surface of the Si.sub.3 N.sub.4 film by pyrogenic oxidation or the like, so that a so-called ON (Oxide-Nitride) film as a composite film of an SiO.sub.2 film and an Si.sub.3 N.sub.4 film is formed as the capacitive dielectric film 17. Thereafter, a polycrystalline Si film 18 is deposited. The polycrystalline Si film 18 is doped with phosphorus by predeposition or the like, and is processed into the shape of an upper electrode.
FIGS. 2A to 2E show the second related art, exemplifying a method of manufacturing a so-called cylindrical type capacitive element constituting the memory cell of a DRAM. In this second related art, an SiO.sub.2 film 22 and an Si.sub.3 N.sub.4 film 23, which serve as interlayer dielectric films, are sequentially deposited on an Si substrate 21 having a transistor (not shown) formed thereon, as shown in FIG. 2A.
Subsequently, as shown in FIG. 2B, a contact hole 24 for a lower electrode is formed in the Si.sub.3 N.sub.4 film 23 and the SiO.sub.2 film 22 so as to reach the diffusion layer (not shown) of the transistor. After the contact hole 24 is filled with a polycrystalline Si plug 25, a BPSG (Boro-Phospho Silicate Glass) film 26 is deposited. Thereafter, as shown in FIG. 2C, a recessed portion 26a matching the shape of a lower electrode is formed in the BPSG film 26. A polycrystalline Si film 27 is deposited and doped with phosphorus or the like.
As shown in FIG. 2D, an SiO.sub.2 film 28 is deposited. The SiO.sub.2 film 28 is etched back using the polycrystalline Si film 27 as a stopper, thereby filling a recessed portion 27a of the polycrystalline Si film 27 with the SiO.sub.2 film 28. The polycrystalline Si film 27 is etched back using the BPSG film 26 as a stopper and the SiO.sub.2 film 28 as a mask, thereby fabricating the polycrystalline Si film 27 into a cylindrical shape.
Next, as shown in FIG. 2E, using the Si.sub.3 N.sub.4 film 23 as a stopper, the BPSG film 26 and the SiO.sub.2 film 28 are removed by hydrofluoric acid or the like, thereby forming a lower electrode consisting of the polycrystalline Si film 27. Thereafter, a capacitive dielectric film and an upper electrode are formed following the same procedures as in the first related art, though the film and electrode are not illustrated.
In the first related art shown in FIGS. 1A to 1D, since the polycrystalline Si film 15 is doped with an impurity at a high concentration, enhanced oxidation takes place. In forming the capacitive dielectric film 17, therefore, even after the native oxide on the surface of the polycrystalline Si film 15 is removed by hydrofluoric acid or the like, another native oxide is formed on the surface of the polycrystalline Si film 15 again during the subsequent rinsing process for the hydrofluoric acid.
For this reason, the Si.sub.3 N.sub.4 film and the SiO.sub.2 film are sequentially formed on the native oxide in fact, and an ONO film is undesirably formed as the capacitive dielectric film 17, though an ON film must be formed as the capacitive dielectric film 17.
As a result, the effective thickness of the capacitive dielectric film 17 increases. With the same effective thickness, then, the leakage current flowing through the capacitive dielectric film 17 increases, and the dielectric life of the capacitive dielectric film 17 becomes shorter. It is therefore difficult to manufacture a reliable capacitive element having a large capacitance. The same problems arise in the second related art shown in FIGS. 2A to 2E as well.